This invention relates generally to systems for processing waste materials, and more particularly to a system for treating sludge dredged from a waterway with an appropriate amount of water to be pumped in slurry form through a floating pipeline.
Much effort has been directed to cleaning up toxic wastes in a drive to restore our natural environment. Some of such toxic waste is in the form of sediment or sludge that lies at the bottom of waterways. These sediments tend to concentrate heavy-metal toxins, halogenated hydrocarbons, pesticides and anaerobic bacteria. Periodically, the sludge is removed by dredging and then transported, either by barge or pumped in slurry form through a floating pipeline, to an upland shore facility for disposal.
Conventional dredging processes generally include three operations: digging, transportation and disposal of the sludge. These processes employ either suction (e.g. hopper or cutter-head) or mechanical (e.g. dipper, ladder or clam shell) digging techniques. The dredgers alone, or with the aid of barges, floating pipelines and conveyors, are able to transport the dredged material to an on-shore site.
In the case of upland disposal, the sludge may be transported via a floating pipeline as a watery pulp or slurry. The concentration of sludge solids is very low; the ratio by weight between the sludge and water is from about 1/7 to about 1/15, or one part of sludge solids to every 7 to 15 parts of water. At the disposal site, the sludge solids are separated from the water and the water is returned to the waterway as an effluent. Because some contaminated sludge remains in the returned water, pollution of the water environment is very likely.
The Hazardous and Solid Waste Amendments (HSWA) to the Resource Conservation and Recovery Act (RCRA), United States Public Law P.L. 98-616 and regulations written by the United States Environmental Protection Agency (EPA) include specific provisions restricting the direct land disposal of many hazardous wastes, including contaminated sludge and waste water. These restrictions, commonly referred to as the “Land Ban,” require that many types of waste including sludge and waste water be treated prior to land disposal to reduce the toxicity of the hazardous components.
The composition of such sludge is, of course, highly variable. When the sludge is transported via a floating pipeline, a large amount of make-up water must be added to produce a slurry with appropriate pumping characteristics. This make-up water is taken from the surrounding waterway and is further contaminated by treatment chemicals during decontamination and separation of sludge solids at an on-shore treatment facility. The decontamination facility may use chemical agents such as chlorine or sulphur dioxide, chemical coagulants and electrochemical flocculation techniques in various combinations to accomplish sludge decontamination. The process water generated by such chemical treatment must be further treated and made environmentally safe before land disposal or return to the waterway.
A slurry processing unit (SPU) as described in U.S. Pat. No. 5,269,635 was designed to deliver an optimum slurry characteristic to the deposition point of a confined disposal facility (CDF) and reduce the total amount of make-up water added to the slurry to the minimum while still maintaining a practical and feasible transportable mixture.
Reduction of make-up water added during the transportation phase was originally accomplished by managing the injection of make-up water received from the surrounding water body with an onboard monitoring and control computer. While this optimizes the relationship between the slurry's geotechnical characteristics, including its viscous properties, it still requires a substantial volume of clear make-up water that must eventually be filtered or treated chemically prior to its release back into the waterway.